Reverse osmosis (RO) membranes are being utilized in water filtration and purification systems found in households, factories and hospitals. The membranes have sub-microscopic pores that are sized to pass water while rejecting bacteria and other contaminants. A properly designed membrane and system allows only desired molecules to pass through the membrane barrier regardless of the feed stream contaminant level.
As biofilm or other contaminants build up on a membrane system, the system performance will deteriorate. For example, the filtration system may require an increased pressure differential to produce the same flux as the system in its “clean” state. Stated otherwise, for the same level of pressure differential, the flux rate of the system will decrease. For these purposes, the term “pressure differential” or “Delta Pressure” refers to the difference in pressure between the feed stream and the permeate stream, and “flux” refers to the flow rate of the permeate stream. For each cycle between periodic cleanings, the flux will gradually deteriorate over time as biofilm and other contaminants build up on the membrane system. The periodic cleaning will cause the flux level to increase, although typically not to its peak level from the previous cycle because the membrane will generally degrade due to use and, in most circumstances, due to the cleaning process. Thus, in addition to the fluctuations of the system flux between periodic cleanings, there will also be an observed general decline in system performance over time.
Biofilms occur in a wide range of locations. Many are found on or in the human body, including on the teeth, gums, ears, prostate, lungs, and heart, where they are believed to be implicated in chronic infections such as gum disease, ear infections, infections of the prostate gland and heart, and lung infections in people with cystic fibrosis. Biofilms also occur in nature, such as the slime that covers river rocks, marshes, and the like. Biofilms also occur in medical equipment, such as catheters and hemodialysis systems, and are a major source of hospital infections. Biofilms can also occur in areas such as contact lenses, other medical equipment, and in other industries. A primary difficulty with biofilms is that they are more difficult to reduce or eliminate than are individual bacteria. This is due to the formation of the protective layer of slime, as well as adaptations that the individual bacteria undergo when they form biofilms.
One important area in which biofilms occur is in aqueous systems that use separation membranes, such as particle filtration, microfiltration, ultrafiltration, nanofiltration, and particularly reverse osmosis (RO) systems. Microfiltration membranes are typically polymer or metal membrane disc or pleated cartridge filters rated in the 0.1 to 2 micron range that operate in the 1 to 25 psig pressure range. Ultrafiltration is a crossflow process that rejects contaminants (including organics, bacteria, and pyrogens) in the 10 angstrom to 0.1 micron range using operating pressure in the 10 to 100 psig range. Nanofiltration equipment removes organic compounds in the 200 to 1,000 molecular weight range rejecting selected salts. Reverse osmosis removes virtually all organic compounds and 90 to 99% of all ions under pressure in the 200 to 1000 psig range.
These systems use membranes to selectively remove or separate extremely small substances from water and process streams in residential, commercial, and industrial applications. When biofilm is present on the membrane due to microbial growth, colloidal solids and insoluble precipitates can adhere to the sticky substance. As this combination builds, water transmission rates through the membrane are reduced and/or additional pressure must be applied to maintain the same water transmission rates. Colloidal solids, microbiological growth and insoluble precipitates can collect on the membrane during operation. Conventional treatment methods include continuous dosing, in which a residual level of a biocidal agent is maintained within the system, or periodic cleaning and sanitization, in which the filtration system is shut down for a periodic cleaning and sanitization using biocidal agents, acids and caustics. Even with continuous dosing methods, at some point the filtration system must be shut down so that the membrane can be cleaned or replaced. This results in downtime and consequent additional operating expense. Moreover, the cleaning and biocidal agents and caustics that are conventionally used to clean and sanitize the filtration systems have the effect of degrading the filter membranes, which are typically comprised of polymers such as cellulose acetate or polyamide polymers. A number of treatment processes are also available to reduce the fouling potential of the feed water being introduced to the membrane. These include various types of filtration, disinfection, and chemical treatment. Even with these methods, however, most RO treatment systems must be cleaned regularly.
More information about biofilms is provided in an article entitled “sticky Situations: Scientists are Beginning to Understand How Bacteria Find Strength in Numbers” by Jessa Netting, published in Science News, 60:28-30, Jul. 14, 2001, which is hereby incorporated by reference herein in its entirety.
The majority of hemodialysis centers in the United States are equipped with reverse osmosis systems that operate with thin film composite membranes. Thin film composite membranes are susceptible to oxidation with certain disinfection chemicals such as chlorine. The use of chlorine will cause hydrodysis of the membrane and thus the membranes will lose their ability to reject minerals, bacteria and pyrogens. The current acceptable disinfectants for these membranes is RENALIN®, glutaraldehyde and formaldehyde, all of which are difficult to handle and are believed to be carcinogenic.
In municipal water thin film composite membranes are installed in reverse osmosis systems to reduce the total dissolved solids and to remove 99.99 percent of the bacteria and pyrogens. The water quality must meet AAMI standards.
U.S. Pat. No. 6,699,391 to Baldridge et al discloses a method for decreasing the growth of biofilm on reverse osmosis membrane utilizing enzymes.
U.S. Pat. No. 6,180,056 to McNeel et al discloses a method of controlling fouling of a reverse osmosis membrane through the use of an anionic antiscalant and a cationically charged biocide.
Application Ser. No. 11/394,805 to Siegel which is herein incorporated by reference discloses compositions for treating biofilm in medical lines which compositions release at least 80 ppm of free molecular iodine that can be used in the present invention.